Method of producing chemical conversion coatings on metal surfaces

ABSTRACT

A METHOD OF PRODUCING A CHEMICAL CONVERSION COATING ON METAL SURFACES USING AN AQUEOUS COATING SOLUTION AND KEEPING THE SOLUTION ACTIVE BY PERIODICALLY ADDING SUPPLEMENTING CHEMICALS COMPRISING CONTACTING THE METAL SURFACE TO BE COATED AT THE BEGINNING OF THE LAYER FORMING TREATMENT WITH A COATING SOLUTION WHICH CONTAINS A MAJORITY OF THE SUPPLEMENTARY CHEMICALS AND AFTER CONTACTING THE METAL PASSING THE UNUSED SUPPLEMENTARY CHEMICALS INTO THE COATING SOLUTION.

United States Patent 3,758,349 METHOD OF PRODUCING CHEMICAL CONVER- SION COATINGS 0N METAL SURFACES Rudolf Engesser, Frankfurt am Main, Germany, assignor to Oxy Metal Finishing Corporation, Warren, Mich. No Drawing. Filed Sept. 24, 1971, Ser. No. 183,695 Int. Cl. C231? 7/08 U.S. Cl. 148-645 Z 2 Claims ABSTRACT OF THE DISCLOSURE A method of producing a chemical conversion coating on metal surfaces using an aqueous coating solution and keeping the solution active by periodically adding supplementing chemicals comprising contacting the metal surface to be coated at the beginning of the layer forming treatment with a coating solution which contains a majority of the supplementary chemicals and after contacting the metal passing the unused supplementary chemicals into the coating solution.

The invention pertains to an improved embodiment for the production of chemical conversion coatings on metal surfaces.

It has long been known that covering layers can be created on metals, especially iron, steel, zinc, aluminum and their alloys, by means of chemical surface treatment. Aqueous coating solutions of specific composition were used for this purpose. The formation of the coating resulted from a chemical reaction between the solution and the metal surface, e.g., an etching attack. Particular industrial significance has accrued to the processes of phosphating, chromating, oxalating and alkaline oxidation.

When these methods are used in practice, it is customary to proceed by producing a coating solution (bath solution) of the desired composition by dissolving the required starting chemicals in it. The workpieces to be treated are then brought into contact with the coating solution by immersion, flooding or spraying. During this time, the desired covering layer formation takes place on the metal surface, whereby the solution becomes depleted of the layer-forming chemicals. In order to compensate for these losses and to keep the coating solution active, supplementary chemicals are added to the bath in the required quantity depending on the amount consumed, i.e., batchwise or continuously, e.g., by means of dispensing pumps.

An alternative method of phosphatizing metallic surfaces is described in French Pat. 1555386.

The unexpected discovery has now been made that covering layers of substantially better characteristics in use can be obtained by first bringing the workpiece to be coated into contact with the chemicals required for supplementing the bath before using the bath solution in its steady-state composition, and then letting the chemicals be essentially transferred into the treatment bath after contact with the metal surface.

Therefore, the object of the present invention is an improved method for the production of chemical conversion coatings on metal surfaces, using an aqueous coating solution and keeping the solution active by supplementing it with the required chemicals, characterized in that at least an essential part of the chemicals required for supplementing the coating solution is placed in contact with the metal surface as a solution at the beginning of the layer-forming treatments, and only then is the coating solution used on the surface, whereby at least the predominant part of the supplementing chemicals used at the beginning of treatment passes into the coating solution after the contact with the surface to be coated.

Patented Sept. 11, 1973 The new embodiment leads to better results in comparison to the known method of operation, since the metal surfaces to be coated into contact with a fresh, more concentrated and thus more active solution at the beginning of layer formation, so that, for example, etching reactions can take place more rapidly and intensely.

The procedure suggested according to the present invention can be carried out using many types of apparatus.

In continuous spraying plants, in which, for example, workpieces hanging on a conveyor (e.g., automobile bodies) are to be provided with a conversion coating (e.g., zinc phosphate coating), an annular pipe provided with nozzles can be placed at the inlet to the layer formation zone. Then, for example, all of the supplementary solution required can be sprayed on the parts which pass through the spray ring by means of a pump. The workpieces are sprayed with the bath solution in the usual manner, so that the remainder of the supplementary solution used at the beginning is washed off of the surface and carried into the bath. However, it is also possible to inject the solution of supplementary chemicals into the first spray ring of the normal spray system by means of an injector nozzle. In the case of this procedure, the supplementation concentrate is already diluted slightly by the bath solution before encountering the metal parts. However, the effect according to the invention is retained.

A preferred means of using the present invention is in the phosphatization of metallic surfaces, especially iron and steel. The concentrated materials or replenishment materials are mixed with the circulating phosphatization bath liquid and sprayed onto the metallic surface through the above mentioned first spray ring. The concentrate present in the bath liquid which is sprayed through the first spray ring ranges from about 0.02% to about 1.5% by volume. A preferred range is from about 0.1 to about 1.0% by volume.

A preferred replenishment concentrate is one that contains 10% Zn; 10.4% nitrate and 31.6% P0,, with a specific gravity of 1.52. These percentages are by weight.

In the case of immersion plants, e.g., for the surface treatment of strip metals, the strips can first be drawn through a preliminary chamber provided with an overflow device, into which the solution containing the supplementary chemicals is dispensed. The overflowing solution then flows into the main bath, into which the strip dips after passing through the preliminary chamber, and can be finally coated with the desired coating.

The method according to the present invention can be used for the treatment of very varied metals, e.g., iron, steel, zinc, aluminum and their alloys, as well as using the most varied types of coating solutions intended for the production of chemical conversion coatings, such as those for phosphation, chromation, oxalation and alkaline oxidation.

The pre-treatment and after treatment of the pieces to be coated can be conducted as usual.

EXAMPLE In a continuous spraying plant, steel automobile bodies were successively degreased, rinsed, phosphated, rinsed and rerinsed. Then the automobile bodies were basecoated using the electro-dip process.

In the phosphation region, the automobile bodies were first treated in the usual manner for 2 minutes at 60 C. with a zinc phosphate solution containing 3 g./l. of zinc, 7 g./l. of P 0 4 g./l. of N0 3 g./l. of B 0 and sufiicient Na such that the ratio of free P 0 to total P 0 was 0.12, and to which 0.3 g./l. of sodium perborate was added. The phosphation bath was then filled up to a constant P 0 content with a solution containing 12.4 wt. percent zinc, 12.9 wt. percent N 27.1 wt. percent P 0 remainder water, and exhibiting a ratio of free P 0 to total P 0 of 0.55. In addition, the bath was supplemented with sodium perborate.

In the treatment of additional automobile bodies, the procedure was then altered corresponding to the suggestion of the present invention, so that all of the supplementary concentrate was now drawn into the first spray tube ring. The sodium perborate was still added to the bath as before. Because of the mixing of bath solution and supplementation concentrate which took place in the first spray ring, the ratio of free P 0 to total P 0 which took place then was 0.25 to 0.30. The automobile bodies were first wetted with this solution for about 20 seconds, then sprayed for about another 100 seconds with the normal bath solution.

The automobile bodies treated according to the present invention exhibited a zinc phosphate layer which adhered more tightly to the metal, was harder and more finely crystalline in comparison to the parts treated in the usual manner. The corrosion resistance of the automobile bodies treated according to the present invention in connection with an electrodip lacquering film was improved by a factor of 2 to 3 in comparison to the corresponding parts, lacquered in the same manner but not treated according to the invention.

What is claimed is:

1. A method of producing a zinc phosphate conversion coating on metal surfaces using an aqueous coating solution and keeping the solution active by periodically adding supplementing chemicals, comprising: contacting the metal surface to be coated at the beginning of the layer forming treatment with a coating solution which contains a majority of the supplementary chemicals by spraying the supplementary solution onto the metal surface; and, after contacting the metal, passing the unused supplementary chemicals into the coating solution.

2. The method of claim 1 wherein the supplementary chemicals are present at the beginning of the layer forming treatment in the coating solution in a range of from about 0.02 to about 1.5% by volume.

References Cited UNITED STATES PATENTS 1,755,391 4/1930 Green et a1 148-615 R RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 148-6.15 R 

